1. Technical Field
This disclosure relates to liquid droplet discharging devices and image forming apparatuses using the same for ejecting liquid droplets, and more particularly to a liquid droplet discharging device and an image forming apparatus using the same in which liquid droplet ejection performance is improved.
2. Description of the Related Art
Recording heads are used in inkjet recording devices functioning as image forming apparatuses such as printers, facsimile machines, copiers, and plotters. A recording head typically includes a nozzle for discharging ink droplets (recording liquid), a liquid chamber (also referred to as a pressure chamber, a pressurized liquid chamber, a discharge chamber, an ink chamber, an ink flow path, etc.) with which the nozzle is in communication, and an actuator for generating energy to apply pressure to ink inside the liquid chamber. By driving the actuator, ink inside the liquid chamber is pressurized so that ink droplets are discharged from the nozzle to record an image. The mainstream method is an ink-on-demand method performed by discharging ink droplets only when it is necessary to record an image.
Recording heads can be categorized according to various methods depending on the type of actuator used for discharging ink droplets. For example, Patent Document 1 discloses the following known methods. In a piezo method, one of the walls of a liquid chamber is a thin oscillating plate. A piezoelectric element is provided corresponding to the oscillating plate, which piezoelectric element functions as an electromechanical transducer. The piezoelectric element deforms as voltage is applied, which causes the oscillating plate to deform, thereby changing the pressure in the liquid chamber. The change of pressure in the liquid chamber causes ink droplets to be ejected. In a bubble jet method (registered trademark), a heating element is provided inside a liquid chamber. The heating element emits heat when a current is received, and the heat generates bubbles. Pressure due to the bubbles causes ink droplets to be ejected. Patent document 2 discloses an electrostatic type. Specifically, one of the walls of a liquid chamber is an oscillating plate. An individual electrode is provided outside the liquid chamber, facing the oscillating plate. An electric field is applied between the oscillating plate and the electrode, which generates an electrostatic force. The electrostatic force causes the oscillating plate to deform, which changes the internal pressure and the volume of the liquid chamber, so that ink droplets are ejected from a nozzle.
There is also a method employing a piezoelectric element as pressure generating means. As described in examples disclosed in Patent Documents 3 and 4, this method typically employs an oscillating plate provided with an island-shaped or stripe-shaped thick part where the oscillating plate engages with the piezoelectric element. By providing the thick part in the oscillating plate, the engagement with the piezoelectric element is facilitated. This prevents adhesives from sticking out and the engagement position from being displaced, which cause inconsistencies in the discharge volume. Accordingly, the ejected ink droplets are evenly formed.
It is necessary to ensure that the displacement of the piezoelectric element is only communicated inside of the liquid chamber. Problems are created if the piezoelectric element directly pushes up parts other than the liquid chamber such as a flow path unit including the oscillating plate or a flow path plate. Specifically, the pressure inside the liquid chamber does not increase and oscillation of the flow path unit is propagated to other units, which causes mutual interference and a significantly adverse effect on steadiness of ejection. Therefore, the piezoelectric element is generally smaller than the liquid chamber and provided inside the liquid chamber.
Patent Document 1: Japanese Laid-Open Patent Application No. H10-100401
Patent Document 2: Japanese Laid-Open Patent Application No. H2-289351
Patent Document 3: Japanese Patent No. 3147132
Patent Document 4: Japanese Laid-Open Patent Application No. 2003-19794
There are requirements for inkjet recording devices to produce high quality images. To meet these requirements, liquid chambers are reduced in size, so that smaller ink droplets can be ejected. Further, to make nozzles eject ink droplets by a finer pitch, liquid chambers are not only reduced in width but also in length. This increases the pressure resonant frequency of the liquid chamber, so that smaller ink droplets are ejected.
However, even if the liquid chamber is reduced in length, it is not possible to simply shorten the piezoelectric element. FIG. 13 is a schematic diagram for describing displacement of a driving unit of a conventional piezoelectric element 500. Displacement of the laminated-type piezoelectric element 500 occurs in a d33 direction, which is the thickness direction. The piezoelectric element 500 is shortened in the longitudinal direction of a liquid chamber, and the entire bottom surface of the piezoelectric element 500 is bonded and fixed to a base 600. Therefore, displacement of the piezoelectric element 500 is obstructed due to inert parts on both edges. Also, an active part of the piezoelectric element 500 is partly obstructed from being displaced in the thickness direction, which active part is supposed to cause displacement of a thick part 420 of an oscillating plate. When the piezoelectric element is shortened, the areas of the inert parts that obstruct displacement are unchanged, while the active part that becomes displaced in the thickness direction is shortened. Therefore, a predetermined amount of displacement cannot be achieved, and conversion efficiency is significantly degraded. Accordingly, the piezoelectric element 500 cannot be made compact in accordance with the size of a liquid chamber 210, as shown in a partial cut-away view of FIG. 14. Even if the liquid chamber 210 is reduced in size, the size of the piezoelectric element 500 cannot be changed much. Thus, if an oscillating plate 400 only contacts the piezoelectric element 500 at a thick part 420, a large area of a diaphragm part (thin layer part) 410 on the side of a flow resistance part 220 does not receive the effects of the piezoelectric element 500. As a result, compliance (reduced restriction) at the flow resistance part 220 becomes excessively large, such that preferable ejecting properties cannot be achieved.